The major ingredients of modern propellants are actually few. They consist of fuels, oxidizers and binders. Of the three basic ingredients of propellants, two, or even three, of these may be contained in the same material. Nitrocellulose (“NC”) is an example of all three, when colloided. The minor ingredients, used to assist or tie together the major ingredients, are more numerous and sometimes more complex. The interactions of these major and minor ingredients, when combined into a practical solid propellant, are especially complex. Such interactions can take place at all stages of manufacture, storage, and use. Controlling such interactions makes solid-propellant technology intricate and expensive.
Solid propellants are typically divided into classes according to their physical state. Such classes include homogeneous (single-base or double-base) and composite. Double-base propellants are further subdivided according to manufacturing method extruded or cast. As used herein, the term “single base propellant” means and includes a composition that includes an energetic binder, such as NC, and at least one additive, such as, a plasticizer, a ballistic modifier, a stabilizer, a flash suppressor. As used herein, the term “double base propellant” means and includes a composition that includes at least one energetic binder, such as NC, and at least one energetic plasticizer, such as a nitrate ester. For example, the double base propellant may include NC plasticized with the nitrate ester nitroglycerine (“NG”).
Typical traditional methods of NC based propellant production include two separate processes, an NC production process and thereafter a propellant production process. In the NC production process, cellulose material is processed to produce NC. The NC may be mixed with prepared nitroglycerin (“NG”) to produce an NC/NG mixture. NC may be used to produce single-base propellant and NC/NG may be used to produce double-base propellant. In the propellant production process, the produced NC or NC/NG is acquired and is separately processed to produce specific propellants.
The NC production process typically includes, in order, the steps of 1) cutting cellulose material; 2) preparing an acid mixture for nitration; 3) nitrating the cut cellulose in the acid mixture to produce NC, wherein the acid mixture converts the cellulose into cellulose nitrate (C6H9O5(NO2), C6H8O5(NO2)2 or C6H7O5(NO2)3); 4) subjecting the NC to an acidic boiling step; 5) refining the NC, wherein the NC is chopped into smaller pieces; 6) poaching the NC smaller pieces; 7) screening the poached smaller pieces, wherein clumps and unsatisfactory material are removed; 8) blending the screened NC with other batches of poached and screened NC; 9) dewatering the blended NC, wherein water is removed, typically via centrifuging; and 10) dehydrating the centrifuged NC to remove water from the NC, wherein the water wet NC is loaded into a blocking press and compressed into a block, thereby forcing water out via pressure, and, thereafter, alcohol may be injected into the block under pressure to displace remaining water, after which, a densely formed block of NC is formed. The blocked NC may be stored for future use. When NC is requested or ordered for propellant production, the NC block, which is not usable in block form, is then subjected to a block breaking step, wherein the large dense block of NC is broken into smaller chunks that can be managed by a mixer. If the particular propellant production requires an NC/NG starting material, NG material is added to the smaller pieces of the NC prior to forwarding.
The propellant production process typically includes, in order, the steps of 1) acquiring the NC material (NC/NG starting material for double-base propellants); 2) propellant mixing, wherein the acquired NC starting material is mixed with various ingredients desired in the final propellant product to form a propellant mixture, for example, stabilizers, flash suppressant, de-coppering agent and rat/burn modifiers; 3) blocking, wherein the propellant mixture is compressed, effecting a removal of air; 4) extruding and cutting, wherein the blocked propellant mixture is then extruded and cut to produce propellant grains; 5) deterrent coating, wherein the grains coated with a deterrent; 6) drying the grains; 7) screening, wherein unsatisfactory grains are removed; 8) batching, wherein the screened grains are mixed with other lots of the same types of grains; and 9) blending and packing, wherein the batched grains are blended with grains of differing types and packaged for storage or delivery of the produced propellant.
In an alternative to the above propellant production process, prior methods include using a process known as the ball propellant process. In the manufacturing of ball propellant, nitrocellulose is dissolved in ethyl acetate containing small quantities of desired stabilizers and other additives. The resultant syrup, combined with water and surfactants, is heated and agitated in a pressurized container until the syrup forms an emulsion of small spherical globules of the desired size. Ethyl acetate distills off as pressure is slowly reduced to leave small spheres of nitrocellulose and additives. The spheres can be subsequently modified by adding nitroglycerin to increase energy, flattening between rollers to a uniform minimum dimension, coating with deterrents to retard ignition, and/or glazing with graphite to improve flow characteristics during blending.
Operations for the NC production process and the propellant production process are separated by a certain distance known as the quantity distance arc, which is dictated by Ammunition and Explosives Safety Standards. This is required to assure that any ignition issues with the energetic components are not transitioned to the next or neighbor operation. Since propellant extrusion operations are generally performed with unattended batch operations and screw extruders specially adapted for extrusion of energetic materials, the cost to operate is very high relative to the processing of inert materials.
The above methods of NC based propellant production, including the NC production process and the propellant production process, are complex, requiring numerous processing steps and separation of operations, labor intensive and require substantive time and expense.
Examples of further prior methods of preparing cellulose material as a propellant component further include those disclosed in:
U.S. Pat. No. 1,590,598, issued to Taylor, which relates to a method of making smokeless powder from organic material including cotton and carbohydrates, such as starch.
U.S. Pat. No. 1,590,594, issued to Taylor, relates to the treatment and conversion of cellulose or cellulosic materials, including cotton and mixtures of silk and cotton.
U.S. Pat. No. 3,218,907, issued to Beal, relates to felted combustible cartridge cases and a process for the preparation of the same.
US Patent Publication 20060180253 relates to a method for manufacturing microcrystalline nitrocellulose to provide an energetic, nitrogen fuel.
Such further methods do not produce particular particle sized and shaped cellulose based propellants having acceptable thermal stability and ultimate nitrogen substitution ranges that are useful to the small arms industry.
To be useful to the small arms industry, it is desirable that cellulose based propellants exhibit acceptable thermal stability and have ultimate nitrogen substitution ranges. Stabilization of propellant material is required to produce propellants that have sufficiently long shelf lives and remain useful. Basic components of smokeless propellants, for example nitroglycerin and nitrocellulose, undergo decomposition under natural aging conditions. The main decomposition products are nitrogen oxides that are a catalyzer of further accelerated decomposition. To prevent autocatalytic decomposition of nitroglycerine or nitrocellulose, substances that react very fast with nitrogen oxides, e.g. stabilizers, having typically been added to propellants. Stabilizers, in a way “absorb” the catalyzer that results in an increase in chemical stability of propellants. Thus a basic safety condition for propellant (ammunition) storage is a control of its stability on a regular basis. As such, propellants are tested against established standards for their chemical stability.
Due to the energetic nature of such propellants and the high expectations of safety and performance, the processes for producing and the standards for storage and performance of NC are highly scrutinized and standardized. Military standards require specific processing and performance requirements for propellants, such as nitrocellulose propellant. NC must pass the stability requirements of MIL-DTL-244B and more specifically the requirements of MIL-STD-286C, Method 404.1.2. MIL-DTL-244B is included in the U.S. military DETAIL SPECIFICATION: NITROCELLULOSE, which covers the requirements, examinations and tests for five grades, four types, and two classes of nitrocellulose for use in propellant, and MIL-STD-286C is included in the MILITARY STANDARD: PROPELLANTS, SOLID: SAMPLING, EXAMINATION AND TESTING, which describes the general methods of sampling, examining, and testing solid propellants.
The above mentioned MIL-STD-286C of MILITARY STANDARD PROPELLANTS, SOLID: SAMPLING, EXAMINATION AND TESTING, Method 404.1.2, also referred to as the German Test, includes stability requirements and protocols that samples of NC must pass to meet military standards. In this test, a dried NC sample is added to the bottom of a special test tube with a piece of Methyl Violet test paper suspended above the NC sample. The test tube is immersed in a heated bath at 134.5° C. and to be considered thermally stable the test paper cannot completely turned salmon pink until 30 minutes have passed. In Method 404.1.2 (Heat Tests (120 and 135.5C)), Procedure section 4.6, the tester is instructed to test the nitrocellulose specimens by examining the test paper after the first 20 minutes, and thereafter at 5 minute intervals, and discontinue the test when the salmon pink end point is attained in any of the papers, then recording the test time. However, it is further stated in section 4.6 that if the violet paper is not completely changed in 25 minutes, but is completely changed in 30 minutes, the tester is to record the time of the test as 30 minutes. As such, samples that have not completely changed in 25 minutes, but do so prior to the 30 minute mark, are deemed to have passed, even though they have not achieved the full 30 minutes in the stability test.
Prior methods fail to offer NC based propellant production processes that allow for consolidated and concentrated nitrocellulose and propellant production processes at a single location and that include fewer processing steps, requiring less time and expense, and that also result in NC propellant exhibiting complete stabilization.
It would be desirable to provide consolidated and concentrated low cost processes that produce particular particle sized cellulose based propellants having acceptable thermal stability and ultimate nitrogen substitution ranges that are useful to the small arms industry. It would further be desirable to provide methods of production of useful propellants of pre-shaped particles of various sizes and shapes which have high nitration substitution and complete stabilization so as to exhibit sufficiently long shelf life. It would further be desirable to provide such methods which utilize a wide range of starting materials, such as materials differing in composition, shape, size and surface area. It would be further desirable that those propellants survive the necessary mechanical stresses, such as high rate impact and high strain rate environment inside a gun cartridge during ignition and combustion.
The above patents and patent publications are herein incorporated by reference in their entireties. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.